Minnesota paleontology and geology, National Park Service paleontology, the Mesozoic, and occasional distractions

Sunday, June 11, 2017

Hoplitosaurus

With the recent coverage of Zuul and the Suncor nodosaur, it seems like a good time for another entry on North American armored dinosaurs. We've already visited with Nodosaurus textilis, Stegopelta landerensis, and Hierosaurus sternbergii. Today's star is another species of similar vintage, Hoplitosaurus marshi. Like our other three subjects, Hoplitosaurus was initially described around the turn of the 20th century from a single armor-heavy specimen found in Cretaceous rocks of the American West. Also like the other three, Hoplitosaurus received barely a blurb for its initial description and had to wait for someone else to spare a little more time and ink.Frederic Augustus Lucas was a museum guy, not a dinosaur guy. Aside from Hoplitosaurus, the only other place he shows up in the Compact Thescelosaurus is as the person who named Dacentrurus, and that's a case of providing a new name for a preoccupied name (Omosaurus). When this is combined with the cloak of obscurity then surrounding ankylosaurs, it is not all that much of a surprise that for his initial go-round with the holotype specimen of Hoplitosaurus, he decided that what he was looking at was a new species of Stegosaurus. He gave it the specific name marshi in honor of O. C. Marsh, in recognition of Marsh's long connection with Stegosaurus and because the type specimen was apparently one of the last fossils Marsh had examined (Lucas 1901). (Presumably Marsh, who had described both Stegosaurus and Nodosaurus, would not have made the same mistake and would have given it a new generic name, if only to pad his stats.)

The type and so far only known specimen of Hoplitosaurus marshi is National Museum of Natural History (USNM) 4752, and you can see several photos of individual bones at the USNM's database. Per Carpenter and Kirkland (1998), USNM 4752 consists of "ribs, caudal vertebrae, partial right scapula-coracoid, parts of both humeri, right femur, various ossicles, plates, and spines", and if you are confused about any of that, don't forget to check the glossary. This fits well with Lucas's determinations back in 1901, although his "nuchal and gular architecture" is him interpreting some of the "various ossicles" as the kind of throat armor known for Stegosaurus. The specimen was found by Nelson Horatio Darton and his well-groomed 1890s mustache. Lucas did not dwell on the provenance, but it came from near Buffalo Gap Station in Custer County, South Dakota, from rocks of the Lower Cretaceous Lakota Formation (Carpenter and Kirkland 1998); in fact, it came from the same site as the type and only known specimen of what came to be known as Osmakasaurus depressus, which Lucas mentioned as an animal "probably related to Camptosaurus". Lucas noted the numerous and varied spines, plates, and so forth, and interpreted Stegosaurus marshi (which he regarded as the latest stegosaur known) as the extreme development of stegosaurian armor. He reported several large pointed or triangular plates but found no big flat Stegosaurus-type plates, which he suggested might mean that S. marshi sported the heavy smaller plates exclusively. Two of the large pieces of armor are figured, both attributed to the tail.

This piece is 370 mm (14.6 in) "high" (Lucas 1901).

Another plate, 308 mm (12.1 in) "high" (Lucas 1901).

Further speculation on the evolutionary progression of Stegosaurus was scuppered the following year, when upon further review Lucas realized he had something much more like Polacanthus than Stegosaurus. Accordingly, he gave "S." marshi the new genus Hoplitosaurus, evoking the shield of the hoplite (Lucas 1902). Lucas did much better the second time: the connection with Polacanthus has proven to be enduring, even after more than a century of further discoveries and research into ankylosaurs. (He also mentioned Acanthopholis, but generally speaking it's best to walk right past Acanthopholis when you come across it.)

And that was where Lucas left things. It fell to Charles W. Gilmore, Describer of Thescelosaurus, to provide a more thorough description, which he did in 1914 as part of a much larger work focused on Stegosaurus. Gilmore began by recapping Lucas and offering a suspicion that some of the recently named North American armored dinosaurs would prove to be the same. He included Hoplitosaurus in Scelidosauridae because this is 1914. The humerus is estimated as 385 mm (15.2 in) long, and the femur is given as 495 mm (19.5 in) long, which gives you an idea of the scale of the animal. Of the six tail vertebrae, Gilmore thought three were too large to belong to Hoplitosaurus, which made him question the association of the other three. Not counting the armor, the femur is probably the most notable of the postcranial bones, in part because it has a well-developed "lesser trochanter" (a flange on the anterior face of the femur near the proximal end, or the top if you're looking at the figure below; quotation marks because it's not the same thing as the real "lesser trochanter"); other ankylosaurs don't have this to the same extent.

The femur, in anterior, posterior, and lateral views. "a" is the flange in question (Gilmore 1914).

Of course, given that this is an ankylosaur, there is an expectation of some pointy bits and/or armored bits, and Gilmore (and Hoplitosaurus) does not disappoint. Gilmore divided the armor into five categories:

simple flattened: basic unadorned rectangular scutes.

rounded ossicle-like: extremely variable bits 10 to 50 mm (0.4 to 2 in) in longest diameter. "Rounded ossicle-like" pieces were the most common, and account for Lucas's "gular" armor. Gilmore observed that many were found associated with the femur and suggested that they formed part of a sacral shield-like structure.

keeled: again, extremely variable in size and shape, from button-like to elongate with an off-center keel. Some had textile-like ventral surfaces, as in Nodosaurus, and some were scrobiculate, as in Stegopelta, but none had projecting spurs, as found in Hierosaurus.

triangular plate-like: large plates similar to those found in Polacanthus, which had been attributed to the tail in that animal; they featured hollow bases and blood vessel impressions, and could be on the order of 210 mm by 175 mm (8.27 in by 6.89 in).

spined: large spined plates, already illustrated by Lucas (1901), of which eight were found and five were more or less complete. Gilmore subdivided them into three groups, depending on how expanded the base was, the shape of the spine, and the presence or absence of grooves. Some of these were compared to spined plates that had been attributed to the torso of Polacanthus.

Regular ol' keeled scutes (category 3) (Gilmore 1914).

Hoplitosaurus settled into general obscurity in the decades following Gilmore's description, but it has made something of a comeback, which it owes to Polacanthus and the concept of a polacanthid group. Ever since Lucas (1902), Polacanthus has been the main point of reference for Hoplitosaurus. From the late 1980s through the mid 1990s there was a fair amount of support for Hoplitosaurus marshi actually being a species of Polacanthus (Blows 1987; Pereda-Suberbiola 1991, 1994), but this has been more or less dropped (Carpenter and Kirkland 1998). However, the two are still noted as having had very similar armor (Carpenter and Kirkland 1998).

Pereda-Suberbiola, J. 1994. Polacanthus (Ornithischia, Ankylosauria), a transatlantic armoured dinosaur from the Early Cretaceous of Europe and North America. Palaeontographica Abteilung A 232(4–6):133–159.